This invention relates to communications systems and more particularly to a system and method for transmitting and receiving two independently timed PCM (pulse code modulation) signals carried on a single quadriphase modulated carrier.
The quadriphase modulation technique is also known as quaternary phase shift keying, QPSK, four-phase modulation, four-level phase modulation and quaternary phase modulation.
As commonly implemented, the input to a quadriphase modulator is either two synchronous bit streams or a single serial bit stream that is divided into two parallel bit streams prior to carrier modulation. The timing for synchronous bit streams must be generated by the same oscillator or by phase-locked oscillators and the bit stream state transitions when they occur must coincide.
In a quadriphase modulated system, the receiver can detect phase shifts resulting from modulation but cannot measure the absolute phase states unless a transmitted phase reference is also provided. A transmitted reference requires additional power and is not commonly used. The receiver is usually designed to operate on the available information in the received signal to unambiguously demodulate and recover the original PCM bit streams.
In the case of synchronous quadriphase systems, unambiguous signal recovery is accomplished by digitally encoding the four possible PCM signal states, e.g., 00,01,10 or 11, into carrier phase changes of say no change for 00, +90.degree. change for 01, -90.degree. change for 10 and 180.degree. change for 11. The receiver then, being able to uniquely detect carrier phase changes, unambiguously decodes them into the original PCM signal states. This type of encoding is known as quaternary differential encoding. Coding of this type is not possible, however, with asynchronous PCM input signals.
Early patents directed to conventional four-phase modulation systems having synchronous inputs include U.S. Pat. Nos. 2,870,431 to D. F. Babcock and 2,905,812 to M. L. Doelz et al.
In U.S. Pat. No. 3,242,262 to C. M. Melas et al., a four-phase modulator is disclosed wherein as an alternative, two separate unrelated channels of information may be applied. Recognition of the two original channels is provided by use of a known preamble; continuous identification is not provided.
A further four-phase modulation scheme having asynchronous data inputs is discussed in an article by Lucio M. Vallese in Microwaves for August 1971 at page 10. No means for providing unambiguous recognition of the asynchronous channels is disclosed.
In our parent application, Ser. No. 225, 823, filed Feb. 14, 1972, an asynchronous quadriphase communications system is disclosed handling mutually asynchronous input signals having a known characteristic, such as a framing bit. PN scrambling sequences are used to obscure the framing bit when the received channels are routed in the wrong sequence. In an alternative embodiment, mutually asynchronous input signals not having a known characteristic are handled by imposing a low index frequency modulation on the clock of each channel.